1. Field of the Invention
The invention relates to the field of glass recycling. The invention more particularly relates to an automated method for recycling mixed colored cutlet glass (i.e., broken pieces of glass of mixed colors and types) into new glass products. According to a preferred aspect of the invention, a computer controlled process is provided whereby a recycler identifies the virgin glass raw materials, the desired target glass properties, the composition of a batch of mixed colored cullet, and the quantity of cutlet to be used in the glass melt, and the computer program determines the proper amounts of raw materials to add to the batch of mixed colored cullet so that recycled glass is produced having the desired coloring oxides, redox agents, and glass structural oxides in the proper proportion. The recycled glass is then used to make glass products such as beer bottles.
2. Description of the Prior Art
Glass recycling involves collecting used, post-consumer glass and reusing it as a raw material for the manufacture of new glass products. A main repository of recoverable glass is glass containers such as beverage bottles and containers for other products. Bulk recycled post-consumer glass suitable for melting into recycled glass articles is known as cullet. The glass cutlet for recycling is generally provided in the form of small pieces of glass.
Recycled containers comprise different colors, especially amber and green, and also colorless or flint glass. There also may be different types of glass in the respective containers, although soda-lime-silica glass, which primarily contains oxides of sodium, calcium and silicon, is the most prevalent. Other waste glass, e.g., off-quality material and scrap from the manufacture of glass products, may also be re-used in the form of comminuted or ground glass cullet.
Approximately ten percent (10%) of municipal refuse is glass, most of which is in the form of discarded containers from beverages, food products and the like. To encourage recycling and minimize waste, there are certain government legislated guidelines to the effect that new glass products should contain a proportion of recycled glass. There is thus a market for cutlet that can be re-used readily. Unfortunately, this normally requires that the glass be sorted by color.
Municipal refuse glass is typically collected at the street, processed at a central location and ground into small particles to provide cullet for use in the manufacture of glass products. Processing can involve, for example, color sorting by hand or by optical techniques and removal of non-glass contaminants by hand, optical techniques, magnetic, eddy current and metal detecting separation techniques. These techniques are not wholly effective for the separation and color sorting of all of the glass. In sorting, for example, it is possible manually, or mechanically by using a color sensing diverter mechanism, to sort glass by color. However, much of the glass is broken in handling and cannot readily be sorted as whole containers, and sorting of smaller pieces is more difficult. A by-product of glass recycling, even when an attempt is made to sort the glass by color, is a quantity of mixed colored pieces.
The color distribution of the glass in post-consumer solid municipal waste, and accordingly, in typical mixed color cullet, varies regionally. A typical color distribution is approximately 65% flint (colorless), 20% amber, and 15% green. To date, mixed colored cullet has had only limited commercial use, and may be used as an aggregate in paving material, landfill cover, or some similar use, but often is discarded in landfills. The mixed colored material is substantially less valuable than color sorted cullet.
Decolorizing techniques are known in the production of flint glass, especially to remove the tint due to iron impurities, which impurities tend to impart a bluish or greenish hue to "colorless" glass. In the manufacture of colorless glass, particularly soda-lime-silica flint glasses, the presence of iron as an impurity in the raw materials has been a serious problem. The presence of ferrous iron (Fe.sup.+2) tends to cause a bluish or blue-green discoloration in the finished glass in addition to decreasing its overall brightness. The economics of glass manufacture are such that it is difficult to provide low cost raw materials free from these iron impurities, and most significant deposits of sand and limestone contain at least trace amounts of various iron salts and oxides.
When the raw materials are melted in the glass batch at temperatures of about 2,600 to 2,900 F (about 1,400 to 1,600 C), significant amounts of iron present are converted to the ferrous (Fe.sup.+2) state under the influence of the prevailing equilibrium conditions. Decolorizers and oxidizers can be added to the glass batch in an attempt to oxidize the ferrous (Fe.sup.+2) iron, thereby forming ferric (Fe.sup.+3) iron, to minimize this glass coloration. Ferric iron (Fe.sup.+3) is a relatively much weaker colorant than ferrous iron.
In U.S. Pat. No. 2,929,675 (Wranau, et al.), a method is disclosed for spinning glass fibers using a fluid molten glass, which glass is optically enhanced by decolorizing the glass to make it more transparent or translucent, so that infra-red rays of the radiant heat supply more readily pass through the glass for heating the spinnerette. In the Wranau method, glass which is naturally greenish is decolorized by the addition of effective decolorizing amounts of such materials as selenium oxide, manganese peroxide, copper oxide or dispersed gold to the molten glass.
In U.S. Pat. No. 2,955,948 (Silverman), a glass decolorizing method is disclosed which continuously produces molten color-controlled homogeneous glass. In the Silverman method, flint (colorless) and other container glass is decolorized by addition to the molten glass of a selenium-enriched frit as a decolorizing agent, as opposed to selenium in its free state mixed with virgin batch raw materials. This is considered to better retain the selenium in the finished goods without vapor loss thereof. Silverman discloses that various commonly used materials for decolorizing flint glass have been tried to eliminate selenium vapor losses without success, such as various selenium compounds, e.g., sodium and barium selenates and selenides, as well as arsenic, by reducing the iron oxide inherently present therein. Silverman discloses that the decolorizing agent preferably comprises frit compositions containing the essential decolorizing agent selenium in its Se.sup.+4 valence state, and also may contain niter and arsenic. In Silverman's method, the decolorizing agent of selenium-enriched frit is added to the molten flint glass and dispersed therein in order to decolorize the glass.
In U.S. Pat. No. 3,482,955 (Monks), a method is disclosed for decolorizing the ferrous (Fe.sup.+2) oxide content of soda-lime glass which naturally contains up to about 0.1% by weight of ferrous oxide. The method of Monks continuously produces decolorized homogeneous glass using a manganese-enriched frit glass as the decolorizing agent. Monks, in particular, provides a method of decolorizing soda-lime glass containing iron as the impurity by utilizing a decolorizing frit glass containing manganese that produces no undesirable coloration of its own and adding the decolorizing frit glass to the molten base glass. Monks teaches that decolorizing frit glass preferably comprises oxidized manganese in the Mn.sup.+2 state (Mn.sub.2 O.sub.3) and in the Mn.sup.+2 state (MnO), which acts as an oxidizing agent to oxidize ferrous iron to ferric iron in soda-lime glass.
Decolorizing to minimize the tint caused by trace impurities such as a small proportion of ferrous iron is a less severe problem than decolorizing or offsetting recycled glass that has been heavily tinted by the addition of tint producing compounds, such as chromium green found in high concentrations in green glass. A sufficient treatment with decolorizing compositions may be difficult to achieve without also affecting the clarity of the glass or causing other quality and manufacturing problems.
In co-assigned related U.S. Pat. No. 5,718,737, a process was described for re-using mixed colored glass cullet to make new and useful glass products. As described more fully below, in the described process one or more of the colors in the mixed colored cutlet is selectively colorized and/or decolorized to render it useful in the manufacture of glass products in one of the other colors. In particular, a batch of mixed color cullet such as recycled municipal waste glass containing a mixture of green, amber and flint (colorless) glass, was selectively decolorized and/or colorized to a desired color with desired properties. For example, the mixed colored cullet was converted to recycled amber colored glass for the manufacture of amber glass containers, such as beer and other beverage bottles, by selectively decolorizing for green and colorizing to achieve an amber tint, thereby minimizing any adverse effect on the appearance of the container due to the relatively dark amber color.
It is desired to develop a technique for automating this process for commercial glass production whereby different batches of broken glass in mixed colors may be readily rehabilitated to provide a material that is substantially as useful for the production of recycled glass containers as sorted amber, green, or flint glass. In particular, it is desired to expand upon the technique described in U.S. Pat. No. 5,718,737 by automating the recycling process and adapting it to conventional commercial glass production processes by specifying the amount of raw materials needed to create glass products with desired properties using different batches of mixed colored cullet. The present invention has been designed to meet this need in the art.